I personally went with the 10D because I already had a Canon 60D and access to t3i's, and wanted to use all the same lenses between two bodies - I wanted a 2nd body for time lapses so I could just waste shutter actuations all day with it, and so I could baby my 60D and K-x around a little more... before deciding which to sell (I'm parting with my K-x unfortunately - because it doesn't have a video mic jack =[ )
The D50 is definitely a beginner level camera, whereas the 10D is a slightly more "intermediate" level camera. Performance wise, they aren't too differen't however.
The 10D has 7 AF points, compared to the D50's 5 AF points
10D has the ability to fine-tune the white balance (kinda unnecessary if you have photoshop)
10D shoots 3 FPS in continuous shooting, D50 only gets about 2.5 or so
10D uses CF cards, D50 uses SD cards
10D has a 6.3 megapixel sensor, compared to the 6.1 on the D50 (so... negligible difference there)
10D has iso range from 100-3200 (H) and D50 goes only 200-1600 (that said, the Nikon has slightly better noise performance at 1600 than the 10D does at equivalent iso - it's still awful haha. Canon is better for bright light)
The LCD's are .2" in difference size wise.. so thats absolutely negligible...
10D uses a slower USB 1.0 for upload, but I really don't feel it's that slow to tell you the truth.. I pulled 1000 shots into this time lapse this morning in about 2 minutes...
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Either way, they are very similar. I'd go with whichever one you find cheaper/in better condition or whichever one you find a better deal on lenses with. Here's Dpreview's side by side comparison on the two.
http://www.dpreview.com/products/co...n_d50&products=canon_eos10d&sortDir=ascending